Low-power multiple-channel fully depleted quantum well CMOSFETs

ABSTRACT

A multiple-channel semiconductor device has fully or partially depleted quantum wells and is especially useful in ultra large scale integration devices, such as CMOSFETs. Multiple channel regions are provided on a substrate with a gate electrode formed on the uppermost channel region, separated by a gate oxide, for example. The vertical stacking of multiple channels and the gate electrode permit increased drive current in a semiconductor device without increasing the silicon area occupied by the device.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor devices andmanufacture, and more particularly, to multi-channel devices.

BACKGROUND OF THE INVENTION

A conventional MOSFET operates by driving current through the channelregion between the source and drain of a device. The conductivity of thechannel region is modulated by the application of a voltage on theconducting gate above the channel surface and insulated from it. Effortsare ongoing within many MOS integrated circuit manufacturing companiesas well as at many universities and government laboratories to improvethe speed and available drive currents with MOSFETs to reduce theirpower consumption, and to improve their reliability and radiationhardness for applications in harsher remote environments, includingspace.

One of the goals in semiconductor processing is to maximize the use ofthe available silicon area. This allows increased miniaturization of theelectronic circuitry. In particular, it is desirable to maximize thedrive current for a given silicon area. This has included devices inwhich multiple gates are provided. For example, dual gate devices inwhich the drive current is doubled for a given silicon area have beencreated. Further improvements in maximizing the drive current for agiven silicon area are desirable.

SUMMARY OF THE INVENTION

There is a need for providing a MOSFET in which the transistor drive isincreased, while reducing gate leakage current and gate capacitance. Dueto the requirements for miniaturization, such as the push towards sub-45nm ULSI (ultra large scale integration). This increase in current driveshould be obtained without an increase in device size or change inlayout design.

These and other needs are met by embodiments of the present inventionthat provide a multiple-channel semiconductor device comprising a firstinsulator layer on a substrate and a first channel region on the firstinsulator layer. A second insulator layer is provided on the firstchannel region. A second channel region is on the second insulatorlayer, and a third insulator layer is on the second channel region. Agate electrode is provided on the third insulator layer.

The present invention thus provides a device that has a gate electrodeand multiple channel regions which allow for increased drive currentwithout an increase in device size.

The earlier stated needs are also met by embodiments of the presentinvention which provide a method of forming a multiple-channelsemiconductor device comprising the steps of forming a stack on asubstrate, this stack including at least two lightly doped channelregions vertically separated from each other and from the substrate byinsulator layers. A gate electrode of the stack is separated from thechannel regions by a insulator layer. An oxide layer is formed on thesidewalls of the gate electrode and source and drain regions are formedthat contact sidewalls of the channel regions. Gate electrode spacersare formed on the oxide liner on the gate electrode.

The methods of the invention allow for formation of a multiple-channeldevice that does not occupy a greater amount of real estate thanprevious devices, but yet provides more drive current than conventionaldevices. The method may find particular utility in sub-45 nmapplications, for example.

The foregoing and other features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a stack formed in accordance with embodiments of thepresent invention after an etching of layers has been performed tocreate the stack.

FIG. 2 depicts the structure of FIG. 1 following a thermal oxidationprocess and a source and drain extension implant, in accordance withembodiments of the present invention.

FIG. 3 depicts the structure of FIG. 2 following a lightly doped silicondeposition and a heavily doped silicon deposition, in accordance withembodiments of the present invention.

FIG. 4 shows the structure of FIG. 3 after a silicon spacer etch hasbeen performed, in accordance with embodiments of the present invention.

FIG. 5 depicts the structure of FIG. 4 following the formation of anoxide or nitride spacer on the gate electrode and silicon spacers.

FIG. 6 shows the structure of FIG. 5 after a source and drainimplantation and the formation of a silicide region in the gateelectrode, in accordance with embodiments of the present invention.

FIG. 7 shows an alternative embodiment of the present invention thatfollows FIG. 2 in a process flow, and in which raised source and drainstructures are formed in accordance with embodiments of the presentinvention.

FIG. 8 depicts the structure of FIG. 7 after formation of an oxide ornitride spacer and silicide region in the gate electrode, in accordancewith embodiments of the present invention.

FIG. 9 depicts layers in an alternate embodiment of the presentinvention prior to etching of the layers.

FIG. 10 shows the structure of FIG. 9 after the gate electrode and thehard mask have been etched, in accordance with embodiments of thepresent invention.

FIG. 11 shows the structure of FIG. 10 after the formation of a thermaloxide liner on the gate electrode and the deposition of gate electrodespacer material over the gate electrode and the hard mask.

FIG. 12 depicts the structure of FIG. 11 following a spacer etching toform gate electrode spacers in accordance with embodiments of thepresent invention.

FIG. 13 shows the structure of FIG. 12 after a dry etch has beenperformed to create the stack, in accordance with embodiments of thepresent invention.

FIG. 14 shows the structure of FIG. 13 following an extensionimplantation process, with a wet etch that recesses the stack and asilicon deposition over the stack, in accordance with embodiments of thepresent invention.

FIG. 15 shows the structure of FIG. 14 following matching of the siliconlayers to form silicon spacers and the formation of a silicide region onthe gate electrode, in accordance with embodiments of the presentinvention.

FIG. 16 depicts the structure of FIG. 15 following the deposition ofnitride or oxide, the etching of the deposited nitride or oxide to formspacers, and source and drain implantation, in accordance withembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses and solves problems related tomaximizing the drive current for a given silicon area in the formationof MOSFETs and other semiconductor devices. This is achieved, in part,by the creation of a multiple-channel device having multiple channelregions formed on a substrate, with insulator layers separating thechannel regions vertically and from the substrate. The gate electrode isprovided on the uppermost channel region, with an insulator layerinterposed between the gate electrode and the uppermost channel region.Source and drain regions are formed vertically to contact the multiplechannel regions. Verticality of the semiconductor device thus formedprovides increased drive current through the multiple channels withoutincreasing the silicon area required for the semiconductor device.

FIGS. 1-6 describe a method of making a multiple-channel device inaccordance with embodiments of the present invention. The descriptionwill discuss certain materials and process steps in an exemplary manner,but it should be recognized that these materials and process steps areexemplary only as other materials and process steps may be employedwithout departing from the scope of the present invention.

FIG. 1 depicts a stack 10 that has been created on a substrate 12 by adry etching of layers that have been previously formed. The stack 10 ofFIG. 1 includes a first oxide layer 14 a, a lightly doped polysilicon(hereafter silicon) layer 15, another oxide layer 14 b, a second lightlydoped silicon layer 15, and a third oxide layer 14 c. The third oxidelayer 14 c forms a gate oxide layer in FIG. 1. The stack 10 includes aheavily doped polysilicon channel 16 and a hard mask 18, on the siliconnitride, for example, or other hard mask material.

In preferred embodiments of the invention, the silicon in each layer 15is doped prior to the formation of the next layer. The silicon layers 15are doped with a first conductivity type, such as p conductivity type,and gate electrode 16 is doped with a second conductivity type, such asn conductivity type. Alternatively, the gate electrode 16 is doped withp type dopants while the silicon in channel regions 15 are doped with ntype dopants. Furthermore, the silicon regions 15 and 16 may compriseother semiconductor materials, such as silicon germanium (SiGe).

Following the formation of the stack 10, a thermal oxidation process isperformed, the results of which are depicted in FIG. 2. For example, thestack may be exposed to temperatures of between about 900° to 1000° forapproximately less than 10 minutes in an environment suitable foroxidation, as is well known. Strict control of the thermal oxidationprocess is necessary, especially in applications such as sub-45 nm ULSIdevices, to prevent the gate electrode 16 from becoming over oxidized.This may readily occur since heavily doped silicon, such as that in thegate electrode 16, allows oxide to grow much faster than in lightlydoped silicon, such as in the channel regions 15.

FIG. 2 also shows the formation of source and drain extensions by sourceand drain extension implants 21. This may be performed in a conventionalmanner.

FIG. 3 shows the structure of FIG. 2 following the sequential depositionof a lightly doped silicon layer 22 and a heavily doped silicon layer24. The layers 22 and 24 are doped with the same conductivity typedopant as in the gate electrode 16. Hence, in the exemplary embodimentbeing described, the dopant is an n-type dopant. Deposition of thesilicon layers 22 and 24 may be performed by chemical vapor deposition(CVD) for example. It is preferred to deposit doped silicon layersrather than attempting to dope by ion implantation the silicon layersafter they have been deposited, since control of the implantationprocess to form a lightly doped region and heavier doped regions isdifficult to control in this arrangement. However, it is also possibleto perform implanting to achieve the desired doping, and certainembodiments of the invention perform such implanting.

A silicon etch is then performed to form silicon spacers 26 that includethe lightly doped regions 22 and the heavily doped regions 24. Thesilicon spacers 26 contact the first and second channel regions 15 butare electrically isolated from the gate electrode 16 by the thermaloxide 20.

FIG. 5 shows the structure of FIG. 4 following the deposition of aninsulating spacer material and a dry etch procedure that forms spacers28 over the silicon spacers 26. The insulating material may be an oxideor nitride or other suitable material, for example.

FIG. 6 depicts the structure of FIG. 5 after the hard mask 18 has beenremoved by etching, for example. A portion of the gate electrode 16 issilicided to form a silicide region 30 by conventional silicidetechniques, including the deposition of a refractory metal layer andannealing to silicidize a portion of the gate electrode 16.

A source and drain implant process is performed in a conventional mannerto create source and drain regions 32 in a substrate 12. The spacers 26,28 serve as a mask during the source and drain implant process.

The arrows in FIG. 6 represent an exemplary electron flow through themultiple channels created in the device of the present invention. Hence,while not increasing the width of the semiconductor device, the multiplechannels (three such channels in FIG. 6 embodiment) provide increasedelectron flow and drive current. Further, gate leakage current and gatecapacitance are reduced.

In certain embodiments of the invention, the gate electrode 16 may beformed of a metal or completely silicidized. The metal gate may be areplacement metal gate, or may be formed initially. Furthermore, one ormore of the insulator layers 14 a-14 c may consist of a high k gatedielectric material, rather than a relatively lower k oxide or othermaterial.

FIGS. 7 and 8 show structures constructed in accordance with anotherembodiment of the present invention. Following the thermal oxidation andsource and drain extension implant steps of FIG. 2, FIG. 7 provides fora raised source and drain formation process. This is performed, forexample, by growing silicon on the substrate 12 in a known manner, andthen etching the silicon to form the raised source and drains 40.

In FIG. 8, an insulating spacer 42 has been formed over the raisedsource and drains 40. Spacers 42 are formed from an oxide or nitride,for example. A silicide region 48 is formed on the gate electrode 16, ina manner as described earlier.

FIGS. 9-16 depict still another embodiment of the present invention thatmay be considered especially preferred since it avoids the risks ofthermal oxidation that are especially present in very small devices.

FIG. 9 shows a number of layers on a substrate 50. The layers includeinsulator layers 52, made of an oxide, for example. Other materials maybe used, such as high K gate dielectrics, for example. Lightly dopedsemiconductor channel regions 54 are provided between the insulatorlayers 52. A gate electrode layer 56 is provided on the uppermostinsulator layer 52. A hard mask layer 58 is provided on the gateelectrode layer 56. As in earlier described embodiments, the gateelectrode layer 56 may be formed from a heavily doped silicon or silicongermanium layer, and the channel layers 54 may be formed of a lightlydoped silicon or silicon germanium material. The conductivity types ofthe dopants in the gate electrode layer 56 and the channel layers 54 areof opposite types. In the described exemplary embodiment, for purposesof explanation and illustration, the gate electrode layer 56 isconsidered to have been heavily doped with n type dopants, while thechannel layers 54 will be considered to have been lightly doped withp-type dopants.

Following the formation of the layers in FIG. 9, a dry etching isperformed to shape the gate, the results of which are depicted in FIG.10. The gate electrode 56 a is thus created, with the dry etchingstopping on the gate insulator layer 52, which forms the uppermostinsulator layer in the stack. The hard mask 58 a is also etched at thistime. A conventional dry etching technique may be employed.

FIG. 11 shows the structure of FIG. 10 after a thermal oxide liner 60has been formed on the gate electrode 56 a to serve as protection forthe gate electrode 56 a. Subsequent to the formation of the thermaloxide liner 60, a second insulator layer, such as nitride, is depositedby CVD, for example.

A spacer dry etch step is then performed, the results of which aredepicted in FIG. 12. The spacer dry etch stops on the top insulatorlayer 52. The dry etch creates gate electrode spacers 64 on the thermaloxide liner 60. A conventional dry etch process may be employed in thisstep.

A second etching procedure is then performed, using a dry etch, tocreate a stack 66. The etching employs the gate electrode spacers 64 asa mask, while insulator layers 52 and channel layers 54 are etched. Thedry etch is stopped on the bottom insulator layer 52 for controlpurposes to prevent damage to the silicon substrate 50.

FIG. 14 shows the structure of FIG. 13 following an extension implant ofthe same type dopants as provided in the gate electrode 56 a. Theimplants create extensions 68 in a substrate 50. Following the extensionimplant, a wet etch process is performed that recesses the stackunderneath the gate electrode spacer 64. It also serves to remove aportion of the bottom insulator layer 52. However, a second wet etch maybe performed to remove more of the bottom insulator layer 52 ifnecessary to cause it to form the structure depicted in FIG. 14. Therecessing of the stack 66, including channel regions 54 a and insulatorregions 52 a, allows the diffusion of the extension implants to reachthe channel under the gate electrode 56 a.

Following the recessing of the stack 66, a multi-step polysilicondeposition process is performed to subsequently deposit a lightly dopedpolysilicon layer 70 over the stack 66, followed by a heavily dopedpolysilicon layer 72 over the lightly doped polysilicon layer. Thedopant conductivity type is the same as in gate electrode 56 a and inthe extension implants 68. In this example, the dopant conductivity typeis n-type dopant. The deposition of the polysilicon layer 70, 72 may beby chemical vapor deposition (CVD) or other appropriate methodologies.

FIG. 15 shows the structure of FIG. 14 following a dry etch of thesilicon layer 70, 72 to form silicon spacers 76, having a lightly dopedregion and a heavily doped region. At this time, silicide can then beformed on the silicon spacers 76 and in the gate electrode 56 a, oncethe hard mask 58 a has been removed by appropriate etching techniques.

FIG. 16 depicts the structure of FIG. 15 following the formation of aninsulator spacer 80, made of nitride or oxide or other suitablematerial, for example. A source and drain implant is then performed tocreate source and drain regions 82 in the substrate 50.

Paragraph 3 separate channel regions 54 b are depicted in the structureof FIG. 16, providing for a total of four channels (including thechannel formed in the substrate 50). Hence, it should be clear to thoseof ordinary skill in the art that the number of channel regions may bevaried in different embodiments.

The present invention thus provides for a semiconductor device andmethod for making the same that has a greater drive current thanprevious devices, but without occupying a greater amount of silicon areathan conventional devices.

Although the present invention has been described and illustrated indetail, it is to be clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the scope of the present invention being limited only by theterms of the appended claim.

1. A multiple-channel semiconductor device, comprising: a firstinsulator layer on a substrate; a first channel region on the firstinsulator layer; a second insulator layer on the first channel region; asecond channel region on the second insulator layer; a third insulatorlayer on the second channel region; and a gate electrode on the thirdinsulator layer.
 2. The device of claim 1, wherein the first and secondchannel regions are lightly doped with a first conductivity type ofdopant.
 3. The device of claim 2, wherein the gate electrode is heavilydoped with a second conductivity type of dopant different than the firstconductivity type.
 4. The device of claim 3, wherein the first andsecond channel regions and the gate electrode are doped silicon.
 5. Thedevice of claim 4, further composing doped silicon spacers on thesubstrate and forming sidewall spacers contacting the first and secondchannel regions.
 6. The device of claim 5, further comprising thermaloxide on sidewalls of the gate electrode.
 7. The device of claim 5,further comprising CVD oxide on sidewalls of the gate electrode.
 8. Thedevice of claim 4, further comprising raised source and drain regions onthe substrate and contacting the first and second channel regions. 9.The device of claim 1, wherein the insulator layers are oxide.
 10. Thedevice of claim 1, wherein at least one of the insulator layers is ahigh K gate dielectric.
 11. The device of claim 1 wherein the channelregions are SiGe.
 12. The device of claim 1, wherein the gate electrodeis doped SiGe.
 13. The device of claim 1, wherein the gate electrode isat least one of a metal or a silicide.
 14. A method of forming amultiple-channel semiconductor device, comprising the steps: forming astack on a substrate, the stack including at least two lightly dopedchannel regions vertically separated from each other and from thesubstrate by insulator layers, and a gate electrode separated from thechannel regions by an insulator layer; forming an oxide liner onsidewalls of the gate electrode; forming source and drain regionscontacting sidewalls of the channel regions; and forming gate electrodespacers on the oxide liner on the gate electrode.
 15. The method ofclaim 14, wherein the step of forming an oxide liner includes forming athermal oxide liner on the sidewalls of the gate electrode.
 16. Themethod of claim 15, wherein the step of forming gate electrode spacerson the oxide liner include depositing nitride on the oxide liner and thegate electrode and performing a spacer etch stopping on the insulatorlayer separating the gate electrode from the channel regions.
 17. Themethod of claim 16, wherein the step of forming a stack includes dryetching the insulator layers and the channel regions after the gateelectrode spacers are formed, stopping the dry etching on the insulatorlayer separating the channel regions from the substrate.
 18. The methodof claim 17, further comprising implanting source and drain extensionsinto the substrate after the dry etching.
 19. The method of claim 18,wherein the step of forming source and drain regions includes depositinga lightly doped semiconductor layer over the stack and the gateelectrode spacers after the implanting of the source and drainextensions, and depositing a heavily doped semiconductor layer over thelightly doped semiconductor layer.
 20. The method of claim 19, whereinthe step of forming source and drain regions further includes dryetching the lightly doped and heavily doped semiconductor layers to formsemiconductor spacers on the sidewalls of the channel regions.
 21. Themethod of claim 20, further comprising forming device spacers on thesemiconductor spacers and performing a service and drain implantationinto the substrate with the device spacers masking the source and drainextensions.